Image processing with openCV in iOS - ios

Question

I'm looking to retain only red colour pixels and darken everything else from the image. And I want to use openCv. I managed to filter red colour using the below code, thanks to SO, detect colors from object and change its color ios
// Create Mat from UIimage
cv::Mat img = [self cvMatFromUIImage:[UIImage imageNamed:#"rgb1.jpg"]];
// Convert to HSV
cv::Mat hsvImage = cvCreateImage(img.size(),8, 3);
cv::cvtColor(img, hsvImage, CV_BGR2HSV);
std::vector<cv::Mat>channels;
// splitting the channels of HSV
cv::split(hsvImage, channels);
// Getting only the hue from channels
cv::Mat hue = channels[0];
// Creating a temporary image using the hue
cv::Mat dest;
cv::Mat temp = cvCreateImage(img.size(), 8, 3);
// Giving the threshold range
cv::inRange(hsvImage, cv::Scalar(90,50,50), cv::Scalar(130,255,255), dest);
// I guess image temp Image and Original image gets merged here
// I would appreciate some explanation here
cv::merge(channels, temp);
temp.setTo(cv::Scalar(90,50,50),dest);
cv::split(temp, channels);
cv::merge(channels, dest);
// Converting the HSV Image back to BGR
cv::cvtColor(dest, hsvImage, CV_HSV2BGR);
// Converting Mat to UIImage
self.imageView.image=[self UIImageFromCVMat:hsvImage];
But I want to keep red colours as it is and darken or blur the remaining colours. I'm confused where should I make those inverse action and how to do it as well.
Any help would be appreciated.
Updated:
Code that worked for me, hope it helps someone out there.
cv::Mat img = [self cvMatFromUIImage:[UIImage imageNamed:#"rgb1.jpg"]];
cv::Mat hsvImage;
cv::cvtColor(img , hsvImage, CV_BGR2HSV);
cv::Mat mask;
cv::inRange(hsvImage, cv::Scalar(90,50,50), cv::Scalar(130,255,255), mask); // This picks red color
// cv::inRange(hsvImage, cv::Scalar(0,50,50), cv::Scalar(30,255,255), mask); // This picks blue color
self.imageView.image = [self UIImageFromCVMat:mask];
cv::Mat maskRgb;
cv::cvtColor(mask, maskRgb, CV_GRAY2BGR);
cv::Mat result;
// cv::bitwise_and(img ,maskRgb ,result); // #berak but app crashed at this line
img.copyTo(result, mask); // This line writes the new masked image over the original image, I'm not sure if thats the right way instead of bitwise_and???
self.imageView1.image = [self UIImageFromCVMat:result];

Answer 1

you probably don't need the split/merge pass. why not start all simple, and make a mask from the hsv image with inRange, and apply that on the image ?
cv::Mat hsvImage;
cv::cvtColor(img , hsvImage, CV_BGR2HSV);
Mat mask; // red is on the left side of the [0..180] hue range
cv::inRange(hsvImage, cv::Scalar(0,50,50), cv::Scalar(30,255,255), mask);
cv::Mat maskRgb; // make a 3channel mask
cv::cvtColor(mask, maskRgb, CV_GRAY2BGR);
Mat result;
bitwise_and(img ,maskRgb ,result);